Drop-on-demand printers are becoming a significant fraction of the market of ink jet printers. These printers differ from continuous jet printers in that the only time the print head ejects ink, in the case of drop-on-demand, is when printing has to take place. Also, the ejection pressure is thermally generated, rather than hydraulically generated.
Such drop-on-demand printers create new demands on inks to be used with them. Chief of these is that the ink must not crust over between actual printing events, since there is insufficient force in thermal generation to blast through the crust. Unfortunately, crusting over is much more likely to occur with drop-on-demand printers, since there is so much greater quiescent time when the print head sits idly by, exposed to atmospheric drying.
It is well known that humectants, including alkanediols such as ethylene glycol, can be added to an ink to prevent drying out or crusting of the ink. Such is commonly done, for example, in continuous jet inks. However, the amounts used in such inks do not exceed about 5 weight percent, since humectants are highly viscous and the viscosity of continuous jet inks cannot exceed about 2.0 centipoise to be effective.
It is well known that the viscosity restraints are much less for drop-on-demand ink. This would seem to suggest that the crusting problem can be solved by loading up the ink with excess, and even majority, amounts of humectants, wherein "majority" means at least 50 wt %. In fact, "excessive" amounts are recommended, in order to provide a high enough viscosity. However, such excessive amounts of humectants such as ethylene glycol, react adversely with many dyes to cause dye precipitation. Dye precipitation, like crusting, is unsatisfactory as it causes plugging of the print head.
The problem addressed by this invention is to find humectant-dye combinations which allow the ink to avoid crusting while at the same time prevent precipitation of useful amounts of the dye. One solution to this problem has been attempted in the ink used in the "Think Jet" print head available from Hewlett Packard, wherein 60 weight % diethylene glycol is present as a humectant. However, in that case the dye incompatibility that causes precipitation has been dealt with only by reducing the concentration of the dye to the point (5 wt % solids concentration) where precipitation does not occur. Such technique is a well-known approach to precipitation problems. Unfortunately, the amount of this particular black dye that is thus present is so small as to provide insufficient coloration. Thus, this black ink ends up being more grey than black. Grey inks are unsatisfactory for text printing since they reproduce poorly or not at all in electrophotographic copiers.